The present invention relates to a structure and a method for mounting a SAW (Surface Acoustic Wave) device to a substrate, particularly with the function surface of the SAW device facing the substrate.
It has been customary to mount a SAW device to a mounting substrate in such a manner as to maintain the function surface of the device airtight and form a preselected space over the oscillation propagation section of the device. With this configuration, it is possible to protect the function surface from extraneous interference. However, a problem with this structure is that a metal cap for implementing airtightness increases the dimension of the mounting substrate in the direction of thickness, obstructing a small size, light weight configuration. Another problem is that a sealing step is necessary and increases the number of production steps. In addition, a ceramic substrate and the metal cap increase the production cost. Although the ceramic substrate and metal cap may be replaced with plastic members in order to reduce the cost, a moistureproof feature available with plastic members is too low to fully protect comb-like electrodes called interdigital transducer (IDT) on the function surface from corrosion. This is apt to deteriorate the characteristic of the SAW device.
Japanese Patent Laid-Open Publication No. 4-150405, for example, teaches a mounting structure using a flip-chip scheme in order to reduce size and weight while enhancing airtightness. This mounting structure not including a metal cap successfully reduces size and weight. However, because the rear or top of a SAW device and its edges are sealed by silicone resin, the entire structure is greater in size than the SAW device itself. Moreover, because the silicone resin covers only the top and edges of the SAW device, temperature variation which may repeatedly act on the entire assembly brings about a stress due to a difference in the coefficient of thermal expansion between the SAW device and a substrate. The stress directly acts on the connecting portions of gold (Au) bumps. As a result, the connecting portions are apt to break due to fatigue and also apt to break when subjected to a shock or impact ascribable to, e.g., a drop. When the connecting portions break, the sealing function of the silicone resin and therefore the characteristic of the SAW device is deteriorated. Although the silicone resin may be replaced with epoxy resin which is less fragile, epoxy resin is likely to enter the gap between the function surface of the SAW device and the substrate due to surface tension, depending on the viscosity of the resin melted at the time of sealing. Such resin deposits on the oscillation propagation section of the function surface of the SAW device and prevents a space for opening the above section from being formed. Consequently, the expected characteristic of the SAW device is degraded.
Technologies relating the present invention are also disclosed in, e.g., Japanese Patent Laid-Open Publication Nos. 60-53058, 3-272212, and 4-369915.